Adhesives can be defined as compositions that join discrete materials or materials of the same nature, conferring to the final assembly new functions and improved properties over those of the isolated components. Herein, the designation “adhesives” encompasses contact adhesives, such as glues and sealants, as well as any and all compositions capable of being applied onto a substrate in the form of a thin film having as a primary function to join surfaces.
Adhesives may present themselves in the form of compositions, usually in the form of a high viscosity liquid solution made of one or more polymers dissolved into solvents and they can also contain additives. Adhesives are broadly used in different industrial fields, such as automotive, aviation, paper, electro-electronics, furniture, leather/shoes and construction. The use of adhesives provides several advantages over other joining processes, such as joining between materials having variable composition and dimension and with any type of surface, reduction of the occurrence of electrochemical corrosion between different materials, thermal and electrical insulation, and dampening of vibration and shock.
Adhesives, especially those classified as solvent-based contact adhesives promote a remarkable reduction in the assembly time and costs over other processes such as welding, weaving or mechanical splice (with rivets, bolts and spikes) used in the industry of wood products in general, shoes, carpets, construction, among others. An example of this type of adhesive is that known in the market as “shoemaker's glue”.
From among the most used polymers in solvent-based contact adhesive compositions, polychloroprene (2-chloro-1,3-butadiene), which is used in the manufacture of synthetic rubbers, can be mentioned. Polychloroprene is obtained by polymerization of the chloroprene monomer and unlike the further elastomers, it can be vulcanized with magnesium oxide. In general, polychloroprene has good chemical stability and retains flexibility over a broad temperature range.
The solvent that is more commonly used in adhesive compositions, especially those containing polychloroprene, is toluene (methyl benzene). Preference for toluene is due to some characteristics of the polychloroprene solution in said solvent, especially high viscosity and slow evaporation after application of a film onto a surface. However, toluene is an aromatic solvent popularly known in Brazil as the toxic component of the shoemaker's glue. Inhalation thereof can lead to addiction and said solvents are known to cause several damages to the organism, especially to the nervous system, causing symptoms such as fatigue, nausea, mental confusion, weakness and loss of memory, appetite, hearing and vision and even loss of consciousness and death.
In order to prevent the problems caused by toluene, novel adhesive compositions employing different solvents have been proposed. An example is Brazilian patent PI9901812-8, which suggests a toluene-free, polychloroprene-containing adhesive composition that uses a cyclic hydrocarbon (cyclo hexane) as a main solvent, but such solvent does not provide the composition with high viscosity and an appropriate drying time at levels similar to those of the toluene-containing compositions.
U.S. Pat. No. 6,469,082 describes low VOC contact adhesive compositions containing polychloroprene solubilized in a blend of solvents comprising approximately 10% cyclohexane, 60% of acetone, 20% of methyl acetate and 10% of toluene. Viscosity levels of compositions containing 20 to 40% solids suggest that the used solvent blend confers high viscosity only as a function of the increase in the amount of solids in the polychloroprene or adhesive solution.
As noted in the aforementioned inventions, the adhesive compositions free of toluene or having a low content of toluene exhibit limitations such as low viscosity of the polychloroprene polymer solution and fast drying of a film of the solution applied onto a surface. Thus, one notes that the field of the invention still allows for one to obtain adhesive compositions with improved formulations.
In experiments intended to assess the solubility of polymers for rheology studies, it has been surprisingly found that the use of a cyclic ketal, 2-methyl-2-ethyl-1,3-dioxolane, was capable of solubilizing polychloroprene. Subsequent tests have shown that this ketal yields adhesive compositions having viscosity and drying characteristics that enable the use thereof as an alternative to compositions containing toluene and further aromatic and oxygenated solvents. The molecule of this cyclic ketal is known but the search for prior-art documents did not show any inventions anticipating the use thereof as a solvent in adhesives.
Thus, it is an object of the present invention to use 2-methyl-2-ethyl-1,3-dioxolane as a solvent in adhesive compositions. It is another object of the invention an adhesive composition comprising 2-methyl-2-ethyl-1,3-dioxolane as a solvent. The advantages of the present invention will be evident from the following detailed description.